Superconducting thin film niobium carbonitrides on carbon fibers

The preparation and characteristics of a new superconducting wire material, consisting of a 7 ..mu..m carbon filament substrate thinly coated with niobium carbonitride, are reported. These filaments are coated (approximately 1000 at a time as a yarn) by a chemical vapor deposition (CVD) process in which the carbonitride is deposited as a compound. NbCl/sub 5/ is the source gas for Nb, either N/sub 2/, (CH/sub 3/)/sub 3/N, (CH/sub 3/)/sub 2/NH, or NH/sub 3/ is the source gas for N, and either CH/sub 4/ or the amines is the source gas for C. Equilibrium thermochemical calculations for the most important reactions were conducted for a range of temperatures. A review is also made of that part of the Nb--N--C phase diagram which pertains to superconducting compositions and the effect of composition on the superconducting transition temperature T/sub c/ is examined. Reaction temperatures between 600 and 1200/sup 0/C were used to produce carbonitrides with transition temperatures up to 16.9/sup 0/K. The effect of CVD parameters on T/sub c/, the width of the transition, and the coating grain size are discussed. It is found that increasing the CVD time or temperature tends to broaden the superconducting transition. Some evidence for slight reaction withmore » the carbon substrate is found.« less